1. No category

Chin. Phys. B Vol. 24, No. 6 (2015) 064203
Electrically tunable holographic polymer templated
blue phase liquid crystal grating∗
He Zheng-Hong(何正红)a)b) , Chen Chao-Ping(陈超平)a)† , Zhu Ji-Liang(朱吉亮)a) ,
Yuan Ya-Chao(袁亚超)a) , Li Yan(李 燕)a)‡ , Hu Wei(胡 伟)a) , Li Xiao(李 潇)a) ,
Li Hong-Jing(李洪婧)a) , Lu Jian-Gang(陆建刚)a) , and Su Yi-Kai(苏翼凯)a)
a) National Engineering Laboratory for TFT-LCD Materials and Technologies, Department of Electronic Engineering,
Shanghai Jiao Tong University, Shanghai 200240, China
b) School of Physical Science and Technology, Southwest University, Chongqing 400715, China
(Received 14 September 2014; revised manuscript received 19 November 2014; published online 10 April 2015)
In this paper, we demonstrate an alternative approach to fabricating an electrically tunable holographic polymer templated blue phase liquid crystal grating. This grating is obtained by preforming a polymer template comprised of periodic
fringes, and then refilling it with a blue phase liquid crystal. Compared with conventional holographic polymer dispersed
liquid crystal gratings, our grating can remarkably reduce its switching voltage from 200 V to 43 V while maintaining a
sub-millisecond response time. The holographic polymer templated blue phase liquid crystal (HPTBPLC) grating is free
from electrode patterning, thus leading to a lower cost and more flexible applications.
Keywords: holographic polymer dispersed liquid crystal, blue phase, phase grating
PACS: 42.40.Eq, 61.30.Mp
DOI: 10.1088/1674-1056/24/6/064203
1. Introduction
Holographic polymer dispersed liquid crystals (HPDLCs)
have been extensively studied due to the high diffraction
efficiency, fast response times, and easy fabrication. [1–5]
They have potential applications in optical gratings, electrooptically addressable volume data storage, diffractive optical elements, [6–8] etc. Electrically switched distributedfeedback (DFB) lasing action was introduced with an
HPDLC grating. [9,10] However, the switching electrical field
of HPDLC is usually of the order of tens of volts per micrometer, which greatly hinders the practical applications of
an HPDLC grating. This is primarily due to the large elastic deformation energy and the strong anchoring force associated with small liquid crystal droplet sizes. [11–13] For this reason, many researchers were dedicated to reducing the switching voltages of HPDLC devices. [11–14] Jang and Kim reported
that the HPDLC is fabricated with the chemically incorporated
graphene oxide, and the switching voltage is reduced from
70 V to 25 V; however, its decay time increases from 3.2 ms
to 4.8 ms. [11] Sarkar et al. reported that the switching field
decreases from 9 V/µm to 2.5 V/µm, by using partially fluorinated monomers, but the decay time increases from 1.6 ms to
18 ms. [12] Klosterman et al. reported that the switching field
decreases from 12.5 V/µm to 5 V/µm, by adding the surfactant, but the decay time is longer. [14] In their work, the switch-
ing voltage was reduced, however, the decay time was longer.
Recently, the blue phase liquid crystal (BPLC) gratings
based on patterned electrodes [15–17] and periodic polymer
slices [18] have attracted substantial attention, because they exhibit high diffraction efficiency, fast response, etc. However,
for BPLC gratings based on patterned electrodes, it is difficult to obtain high resolution because of the fringing electric
field effect. [18] For BPLC gratings based on periodic polymer
slices, the fabrication is relatively complicated.
In this paper, we demonstrate a holographic polymer templated blue phase liquid crystal (HPTBPLC) grating, which
is obtained by replacing the nematic liquid crystal droplets
with a BPLC composite in a conventional HPDLC grating.
This HPTBPLC has a significantly reduced switching voltage
(43 V) compared with the HPDLC grating (200 V), and the
response time remains fast (< 1 ms). The fabrication of an
HPTBPLC grating is free from electrode patterning, thus it
could easily achieve high resolution and large diffraction angle at low cost.
2. Experiments
The fabrication process of making an HPTBPLC transmission grating is depicted in Fig. 1, where four stages are
involved.
As shown in Fig. 1(a), at Stage 1, a uniform mixture fills
∗ Project
supported by the National Basic Research Program of China (Grant No. 2013CB328804), the National Natural Science Foundation of China
(Grant No. 61307028), the Funds from the Science and Technology Commission of Shanghai Municipality (Grant Nos. 11JC1405300, 13ZR1420000, and
14ZR1422300), and the Fundamental Research Funds for the Central Universities, China (Grant No. XDJK 2011C047).
† Corresponding author. E-mail: [email protected]
‡ Corresponding author. E-mail: [email protected]
© 2015 Chinese Physical Society and IOP Publishing Ltd
http://iopscience.iop.org/cpb　http://cpb.iphy.ac.cn
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Chin. Phys. B Vol. 24, No. 6 (2015) 064203
a cell via capillary action. The liquid crystal cell is formed by
assembling two ITO glasses, with a gap held by spacers and
the cell thickness d of 20 µm. In the experiments, the mixture comprises 21.9 wt%:7.5 wt%:1 wt%:0.6 wt%:69 wt% of
TMPTA monomer (Aldrich): N-vinylpyrrollidone (Aldrich):
N-phenylglycine (Aldrich): Rose Bengal (Aldrich): Nematic
liquid crystal (BP-06, ∆n = 0.158, niso = 1.56, ∆ε = 34.2,
Jiangsu Hecheng Co.), and is made uniform by stirring in the
dark.
As shown in Fig. 1(b), at Stage 2, the liquid crystal cell is
first subjected to a holographic exposure to form an HPDLC
grating. Two beams intersect inside the cell and produce an
intensity distribution of the interference fringes. Two beams
are derived from a diode solid-state laser (488 nm, 100 mW,
Coherent) and set to be p-polarization. The intensity distribu-
tion at the cell can be expressed as I(x) = I0 [1 +V cos(𝐾 · 𝑥)],
where I0 = 10.0 mW/cm2 is the total exposure intensity, V is
a constant of fringe visibility and usually V = 0.90–0.95, [19]
𝐾 is the grating vector with an amplitude K = 2π/Λ , and Λ
is the grating period. The beam diameter D is 2.0 mm. A
polymerization preferentially occurs in bright regions, creating a monomer concentration gradient between bright and dark
regions, which causes the diffusions of monomers and liquid
crystal molecules and then polymer-rich or polymer-poor regions appear. [19] An He–Ne laser (λ = 632.8 nm) beam, which
is also set to be p-polarization with an incident angle being
normal to the cell, is employed as the probe beam. The holographic grating is recorded for 20 s at an incident angle of 1.2◦
for each beam. Then, the cell is exposed to ultraviolet (UV)
light to stabilize the HPDLC grating for 5 min.
Stage 1
: monomer
Stage 2
:
: nematic LC
nematic LC droplet
: polymer network
: alternative bright and dark interference
fringes
(b)
(a)
Stage 3
Stage 4
III
II
I
II
: blue phase LC
I : polymerrich region II : polymerpoor region
III : polymerfree gap
(d)
(c)
Fig. 1. (color online) Schematic diagrams of the fabrication procedure. (a) Stage 1: a mixture of monomers and liquid crystals fills
the cell; (b) Stage 2: the HPDLC grating is formed under the alternating bright and dark interference fringes, then illuminated by
an ultraviolet light to be stabilized; (c) Stage 3: the cell is immersed in acetone to wash out the liquid crystal and un-polymerized
residuals. The remaining polymer template consists of the alternating polymer-rich and polymer-poor regions, which are denoted as
regions I and II, respectively. A polymer-free gap is on the top of the polymer template, which is denoted as region III due to shrinking;
and (d), Stage 4: the polymer template is refilled with a BPLC.
As shown in Fig. 1(c), at Stage 3, the polymer template
is formed. The cell is immersed in acetone for 24 h. This
could remove the liquid crystal droplets and non-polymerized
residuals by diffusion. [20] When the cell is moved out of acetone, any acetone remaining in the cell evaporates quickly
at room temperature. The remaining polymer template consists of the alternating polymer-rich and polymer-poor regions,
which are denoted as regions I and II, respectively. After the
polymerization, a polymer-free gap is formed on the top of
the polymer template due to shrinking, which is denoted as region III. The scanning electron microscopy (SEM) images of
the polymer template are shown in Figs. 2(a)–2(d). The grat-
ing period Λ ≈ 14 µm, as shown in Figs. 2(a) and 2(b). The
black and bright rectangle regions are located in the centers of
polymer-poor and polymer-rich regions, respectively as shown
in Fig. 2(a). The SEM images for black and bright rectangle
regions are shown in Figs. 2(c) and 2(d), respectively. The
statistical data for the black and bright rectangle regions are
shown in Figs. 2(e) and 2(f), respectively. The average pore
sizes in black and bright rectangle regions are 189.3 nm and
28.9 nm, respectively. The maximum sizes in the black and
bright rectangle regions are 325 nm and 73 nm, and the minimum sizes in the black and bright rectangle regions are 83 nm
and 13 nm, respectively, as shown in Figs. 2(e) and 2(f).
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Chin. Phys. B Vol. 24, No. 6 (2015) 064203
(a)
(b)
10 μm
10 μm
(c)
(d)
100
nm
100nm
100 nm
(e)
30%
30%
(f)
20%
20%
10%
10%
0%
6996
123150 177204 231258
285312
96123
150177
204231 258285
312330
0%
1016
2329
3643
4956
6269
1623
2936
4349
5663
6976
Pore size/nm
Pore size/nm
Fig. 2. (color online) SEM images and statistical data for the polymer template at Stage 3: (a) cross-sectional view of the polymer
template, (b) top view of the polymer template, (c) cross-sectional view of the white rectangle region in the polymer-rich region, (d)
cross-sectional view of the black rectangle in the polymer-poor region, (e) pore size distribution of the white rectangle region, and (f)
pore size distribution of the black rectangle region.
50 μm
Fig. 3. (color online) Platelet textures observed with the optical polarizing microscope for the HPTBPLC grating at Stage 4 at 70.5 ◦ C.
As shown in Fig. 1(d), at Stage 4, a BPLC fills the cell.
The polymer template can undergo the reversible shrinking
and swelling. [21–24] Because the polymer network is porous,
a liquid crystal could be refilled into those voids, and region
III could disappear due to re-swelling. The BPLC used is a
mixture composed of the same nematic liquid crystal (96.5%
BP-06, Jiangsu Hecheng Co.) for the HPDLC grating and the
chiral dopant (3.5%, R5011, Jiangsu Hecheng Co.). Then the
BPLC first fills the cell via capillary action at 80 ◦ C, and then
cooled down at a rate of 0.1 ◦ C/min. The BPLC could form in
polymer-poor region due to the relatively large pore sizes. [25]
The temperature range for the BPLC in the HPTBPLC is from
69 ◦ C to 72 ◦ C. The morphology of the HPTBPLC grating
is shown in Fig. 3, where the self-assembled BPLC shows a
characteristic platelet texture based on the polymer template
when it is observed in the reflective mode of an optical polarizing microscope. The different colors originate from different
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Chin. Phys. B Vol. 24, No. 6 (2015) 064203
orientations of cubic lattices. [26–31]
3. Results and discussion
τ=
20
HPDLC
HPTBPLC
16
12
8
4
0
40
80
120
160
2
γ1 PDTC
k (2π)2
,
(1)
where γ1 denotes the rotational viscosity of a BPLC, k is the
elastic constant, and PDTC is the pitch length for a BPLC. From
Eq. (1), the decay time is proportional to the square of the
double twist cylinder pitch PDTC , which is determined by the
concentration of chiral dopant. [39,40] Usually, if the concentration of chiral dopant is appropriate, a BPLC can exhibit a
submillisecond graylevel (< 1 ms). In this work, with a 3.5%concentration of chiral dopant R5011, the HPTBPLC has exhibited a slightly slower response than the HPDLC. The response time of the HPTBPLC could be further improved by
increasing chiral dopant concentration (decreasing PDTC ), but
the tradeoff is the increased voltage. [39]
Transmittance/arb. units
Diffraction efficiency/%
Diffraction efficiencies versus applied voltages of
HPDLC and HPTBPLC gratings are plotted in Fig. 4. The
diffraction efficiency η is calculated as a ratio of the diffracted
power of the first order to the total output power. For the
HPDLC grating at Stage 2, η = 15% when the voltage is
0 V, the threshold voltage is 40 V and the switching voltage is 200 V at room temperature. For the HPTBPLC grating at Stage 4, no threshold voltage is observed, η = 14.7%
when the voltage is 0 V and η ≈ 0% at 43 V at 70.5 ◦ C. The
relatively low diffraction efficiency for the HPTBPLC comes
from the small refractive index contrast. The refractive index
of the polymer matrix is 1.527, and niso = 1.56 for BP-06.
If niso could be increased to 1.59, the maximum phase delay
can reach π, and the first-order diffraction efficiency is 33%.
Moreover, if the grating period is reduced or the cell thickness
is increased, the HPTBPLC grating could become a volume
grating possibly with a higher diffraction efficiency. [32–35]
force strength, etc. [36,37] The HPTBPLC grating can maintain
the fast response time because the BPLC can exhibit a fast response time. [26,27,31,39–41] Based on the Gerber model, [38] the
decay time in a BPLC system can be expressed as
2.5
2.0
1.5
1.0
0.5
0
200
Voltage/V
HPDLC
HPTBPLC
0
10
20
30
Time/ms
40
50
Fig. 4. (color online) Experimental results for the first-order diffraction
efficiencies of HPDLC grating (triangles) and the HPTBPLC grating
(squares), respectively.
Fig. 5. (color online) Measured response times of the HPDLC (dashed
curve) and HPTBPLC (solid curve) gratings.
The response times are also measured for the HPTBPLC
and HPDLC gratings, and shown in Fig. 5. The cell is driven
by a square-wave voltage with 1-kHz frequency. The dashed
curve shows the first order transmittance of the HPDLC grating when it is switched between 0 V and 200 V. The solid curve
shows the first order transmittance of the HPTBPLC grating
switched between 0 V and 43 V. The rise and decay time are
defined as 10% to 90% and 90% to 10% of the transmittance
change, respectively. As shown in Fig. 5, the rise and decay
time measured at room temperature are 341 µs and 424 µs
for the HPDLC grating, respectively. While for the HPTBPLC grating, the rise and decay time measured at 70.5 ◦ C are
610 µs and 890 µs, respectively.
The response times of both HPTBPLC and HPDLC are of
the order of submillisecond but governed by different mechanisms. For HPDLC, typical response time varies from 25 µs
to 1000 µs, depending on liquid crystal droplet size, elastic
The reduced switching voltage for the HPTBPLC is attributed to the weakened anchoring force and zero threshold
voltage. First, the anchoring force is weakened in the HPTBPLC grating compared with that in the HPDLC grating. To
verify the anchoring force change, another experiment is carried out by refilling the same liquid crystal at Stage 4 but
without chiral dopant. The switching voltage is reduced from
201 V to 46 V, and the decay time is increased from 450 µs
to several minutes due to the weakened anchoring force in
polymer-poor regions after the wash-out process. [42] Second,
there is no threshold voltage for the HPTBPLC. The HPTBPLC grating can be driven even with a very low operation
voltage; however, for the HPDLC grating, the threshold voltage is 40 V as shown in Fig. 4. Moreover, the driving voltage for the BPLC without polymer stabilization is relatively
low. [41,43] Therefore, the driving voltage for the HPTBPLC is
reduced significantly.
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Chin. Phys. B Vol. 24, No. 6 (2015) 064203
The on-state transmittance of the HPTBPLC grating
keeps increasing. The electro-optic response of a BPLC without polymer stabilization is fast with local director reorientation, but quite slow in the electrostriction region, at approximately dozens of milliseconds. [39] As a result, it takes a little
longer time for some liquid crystals to restore to their original arrangement after the applied voltage has been removed.
The transmittance fluctuation at off-state of HPTBPLC results
from a poor dark state, because the electric field can affect
the blue phase structure and stability. [44] As the electric field
increases, it is possible for a BPLC that the local director reorientation, electrostriction, and field-induced phase transition
are distinguished. [39,41,44]
The simulation work is performed to fit the experimental data. The BPLC consists of countless double-twist selfassembled liquid crystal cylinders stacked in three dimensions,
forming a cubic symmetry. This structural symmetry induces
the optical isotropy of a BPLC when the voltage is 0 V. The
first-order diffraction efficiency can be expressed as [32]
2πd (nI −nII )
η = J21
,
(2)
λ
where J21 is the first-order Bessel function of the first kind, d is
the thickness of the grating, and nI and nII are the average refractive indices of the polymer-poor and polymer-rich regions,
respectively, nI and nII are determined by the Lorentz–Lorenz
relation: [45]

2
n2 −1

n2 −1

 nI −1 = φPI p +φ LCI lc ,

 n2 +2
n2P +2
n2lc +2
I
(3)
 n2 −1
n2p −1

n2lc −1

II

= φPII 2 +φ LCII 2
,
 2
nII +2
nP +2
nlc +2
where np is the refractive index of the polymer network,
nlc is the refractive index for the liquid crystal with an applied field, and the electric field is set to be perpendicular to
the cell. Based on our previous work, [46] the average volume fractions of the polymer network in polymer-rich and
polymer-poor regions can be estimated to be φPI = 91% and
φPII = 66%, respectively. The liquid crystal’s volume fraction
in the polymer-rich region, φLCI , equals 1 − φ PI , and that in
the polymer-poor region, φLCII , equals 1 − φ PII . According to
the experimental results (i.e., squares), when the applied voltage is 0 V, the diffraction efficiency is around 14.7%, arising
from the refractive index contrast between polymer-rich and
polymer-poor areas, and we obtain nI = 1.530 and nII = 1.538
from Eqs. (2) and (3). When the applied voltage increases, nII
decreases due to the decreased nlc of the BPLC in polymerpoor regions. When the voltage reaches 43 V, nII matches nI ,
and the diffraction efficiency drops to zero.
4. Conclusions
In this paper, we demonstrate a holographic polymer templated blue phase liquid crystal grating. This device exhibits
a significant improvement in lowering the switching voltage
from 200 V to 43 V, while maintaining a sub-millisecond response time relative to an HPDLC grating. This promising
approach could bring potential applications in optical switch,
optical storage, electrically switchable lasers, etc.
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